Applicant's U.S. Pat. No. 5,142,712 describes a vacuum assisted toilet, wherein a vacuum is applied to the toilet bowl outlet during a flushing so that less water is required for a complete flushing. The toilet bowl outlet includes a lower trap, in addition to the usual upper trap, with a trapway extending between them. A vacuum is applied to the trapway near the beginning of a flushing. The lower trap has a lower wall that contains a pool of water, and has an upper wall spaced above the top of the pool to leave a gap between them that forms an air passage. Between flushings, the air passage must be open, or else all water in the toilet bowl might be siphoned out. On the other hand, soon after the beginning of a flushing, the lower trap must be closed against the passage of air, so that the vacuum applied to the trapway can help pull out the contents of the toilet bowl, instead of having the vacuum dissipated to the drain or sewer system.
Although the vacuum assisted toilet described in applicant's Pat. No. 5,142,712 operates fairly well, applicant has experimented with the lower trap in an attempt to obtain even better flushings. A better flushing is one which produces a more complete evacuation of the contents of the toilet bowl, using the same or less flush water. In such tests, items that both sink and float in water, such as marbles and plastic balls of different densities, are placed in the toilet bowl and the percent of these items which are removed, is measured. Better flushing might be obtained when the gap above the pool of water is smaller, but a gap of substantial height such as one centimeter (0.4 inch), is necessary to assure that air can pass through the top of the lower trap under worse conditions. Worse conditions would include cases where the toilet is mounted at an incline from the intended orientation with respect to gravity and/or manufacturing tolerances that result in a gap of reduced height. A lower trap which produced better flushings, would be of value.